EFFECTS OF PARASITIC INFECTIONS ON CLUTCH SIZE OF LESSER SNOW GEESE FROM A NORTHERN BREEDING COLONY

A density-dependent decline in the average clutch size of lesser snow geese (Anser caerulescens caerulescens) occurred from 1973 to 1989 at the breeding colony on the shores of La Perouse Bay, Manitoba. An incr ease in average parasite load was hypothesized to be one of the two mo st likely causes of this decline. We shot 28 incubating adult female l esser snow geese at the La Perouse Bay colony and examined the carcass es for parasites to determine if there was any proximate association b etween parasitic infections and the size of the clutch a female laid. We found no convincing evidence that parasitic infections were the pro ximate cause of any reduction in clutch size. In the absence of eviden ce of any direct effect of parasites, we conclude that an increase in the average parasite load is probably not the cause of the long-term d ecline in clutch size at La Perouse Bay. By default, we suggest that i ncreased intraspecific competition for food at the staging areas on th e migratory flyway is the most likely cause of the decline in clutch s ize.